The partial oxidation of aqueous slurries of solid carbonaceous fuel for the production of synthetic gas, reducing gas, and fuel gas is a well known process, such as described in coassigned U.S. Pat. Nos. 3,607,157; 3,764,547 and 3,847,564, which are incorporated herein by reference. A control system with valves in the feedlines for controlling the feed to a gas generator is described in coassigned U.S. Pat. No. 4,479,810. The hot raw process gas stream from the gasifier is quench cooled and scrubbed with water to remove carbon-containing particulate matter that is entrained in the raw gas stream. Aqueous slurries of the particulate matter ground with fresh raw solid carbonaceous fuel and recycled to the gas generator are described in coassigned U.S. Pat. No. 3,607,157.
The Texaco coal gasification process produces three solids-containing streams. These are: coarse slag, fine slag and settler underflow. Much data collected from pilot unit test runs indicate that the fine slag and settler underflow streams contain higher carbon contents than the coarse slag stream. Therefore, the fuel value of these streams may be significant, particularly for petroleum coke gasification where carbon conversions are low. Additionally, the settler underflow stream is contaminated with process water. This process water contains formates, cyanates, dissolved heavy metals and other contaminates that may give rise to problems with permitting the disposal of the settler underflow stream. Therefore, from both an efficiency and environmental standpoint, it is desirable to recycle the fine slag and settler underflow. In the past, solids recycle schemes have involved controlling the flow rate of the recycle solids streams through a control valve. The experience gained with coal gasification units is that the settler underflow stream is highly abrasive and destroys control valves after a short period of operation. Another problem with past recycle solids schemes is that the control of the system depends on on-line density measurements by density meters. The experience gained is that density meters are good for trending purposes but will not be accurate enough for control purposes.
By the subject invention, an improved method for producing an aqueous slurry having a controlled solids content has been developed which has the following advantages over previous concepts:
1. No control valves are used to control the solids recycle stream. As stated above, these valves are failure prone. PA0 2. No density meters are used to control the process. As stated above, density meters are not sufficiently accurate for control purposes. PA0 (1) introducing the solid carbonaceous fuel feed directly into a size reduction zone, wherein weigh belt feeding means controls the feed rate of the solid carbonaceous fuel feed and there is no valving means in the flow path between the weigh belt feeding means and the size reduction zone; PA0 (2) periodically measuring the weigh belt feeder speed and response thereto providing a signal corresponding to the feed rate for the solid carbonaceous fuel in (1) on a weight basis; PA0 (3) periodically determining the weight fraction of moisture in the solid carbonaceous fuel in (1) and generating a signal responsive thereto; PA0 (4) pumping an aqueous slurry of recycle carbon-containing particulate solids directly into said grinding means with no valving means in the line; PA0 (5) periodically measuring the speed of the pump in (4), and responsive thereto providing a signal corresponding to the volumetric feed rate of said slurry of recycle particulate solids; PA0 (6) periodically determining the weight fraction of recycle particulate solids in the slurry in (4) and generating a signal responsive thereto; PA0 (7) periodically measuring the temperature of the slurry in (4) and as a function of said temperature providing a signal corresponding to the density of water at said temperature; PA0 (8) periodically determining the density of the particulate solids in (4) and generating a signal responsive thereto; PA0 (9) automatically computing a value representing the desired rate of flow for the make-up water to be introduced into said size reduction zone in order to provide a slurry of desired solids concentration from the signals generated in (2), (3), (5), (6), (7), (8), and direct current voltage input signals including a signal representing said desired slurry solids concentration; and responsive thereto providing a related signal to a flow recorder rate controlling means which provides an adjustment signal to a valve in the make-up water line, thereby providing make-up water with the desired rate of flow; and PA0 (10) grinding together said solid carbonaceous fuel feed from (1), slurry of recycle particulate solids from (4), and make-up water from (9) in said size reduction zone to produce an aqueous slurry with said desired solids concentration; and introducing said slurry into the partial oxidation gas generator as the fuel feed.